The present invention relates to a disc recording medium such as an optical disc, a disc-manufacturing method for manufacturing the disc recording medium, a disc drive apparatus for driving the disc recording medium, and a reproduction method for reproducing data from the disc recording medium. More particularly, the present invention relates to a disc on which tracks are each wobbled as a pregroove.
As a technology of recording and reproducing digital data, there has been developed a technology of recording data used in recording media onto optical discs including magneto-optical discs. An optical disc can be designed as a CD (Compact Disc), an MD (Mini-Disc), or a DVD (Digital Versatile Disc). The optical disc is a generic name of a disc-like metallic thin plate serving as recording media from which data is read out as changes in reflected laser beam resulting from reflection of a laser beam radiated to the recording media.
To put it in more detail, an optical disc can be of a read-only type or a writable type allowing user data to be written onto the disc. Reproduction-only optical discs include a CD, a CD-ROM, and a DVD-ROM. On the other hand, writable optical discs include an MD, a CD-R, a CD-RW, a DVD-R, a DVD-RW, a DVD+RW, and a DVD-RAM. Data is recorded onto a writable disc by adopting, among other techniques, a magneto-optical recording technique, a phase-change recording technique, and a dye-film-change recording technique. The dye-film-change recording technique is also referred to as a write-once recording technique whereby data can be recorded onto the optical disc only once, and once data has been recorded onto a disc, data can no longer be recorded onto the same disc. Thus, the dye-film-change recording technique is suitable for a recording operation to save data. On the other hand, the magneto-optical recording technique and the phase-change recording technique are adopted in a variety of applications including operations to record various kinds of content data such as musical data, video data, games, and application programs.
In order to record data onto a disc to which the magneto-optical recording technique, the phase-change recording technique, and the dye-film-change recording technique are applicable, a guiding means for tracking a data track is required. For this reason, grooves are created in advance as pregrooves. The grooves and lands are used as data tracks. A land is a plateau-like member sandwiched by two adjacent grooves.
In addition, it is also necessary to record address information so that data can be recorded at any predetermined position on a data track. In some cases, however, the address information is recorded by wobbling the grooves.
Assume that a track for recording data is created in advance as a pregroove. In this case, the side walls of the pregroove each have a wobbled shape representing address information.
By having such a pregroove, an address can be fetched from wobbling information obtained in recording and reproduction operations as information conveyed by a reflected beam. Thus, data can be recorded onto or reproduced from a desired location without creating for example pit data showing addresses in advance.
By adding address information as a groove wobbling shape in this way, it is no longer necessary to provide for example discrete address areas on the track and record addresses in the address areas typically as pit data. Thus, portions for the address areas can be used for storing actual data so that the storage capacity can be increased.
It is to be noted that absolute-time information and address information, which are each expressed by the groove wobbling shape as such, are referred to as an ATIP (Absolute Time In Pregroove) and an ADIP (Address In Pregroove) respectively.
By the way, in the case of a rewritable disc in particular, there may be a situation in which the manufacturer wants to ship a disc containing various kinds of shipping-time information recorded onto the disc in advance. The shipping-time information of a disc is prerecorded information recorded onto the disc in advance prior to the shipping of the disc.
Typically, the shipping-time information includes disc information and system information. The disc information typically includes a recording linear velocity and a laser-power recommended value. On the other hand, the system information shows how to exclude an apparatus of a hacker.
The shipping-time information must be reliable, must have a large size to a certain degree, and must not be falsified.
If the shipping-time information is not reliable, that is, if the disc information included in the shipping-time information is not accurate, for example, there may be raised a problem such as inability to obtain a proper recording condition in the apparatus on the user side.
In an operation to record content data, the data may be encrypted for protection of a copyright. If a key used for encryption is not obtained accurately from the system information, the encrypted data cannot be decrypted so that the content cannot be utilized. This is also because the content data cannot be encrypted in an operation to record the data.
For the reasons described above, disc information and system information, which are recorded as shipping-time information, are required to have reliability higher than recorded and reproduced user data.
The shipping-time information has a large size to a certain degree because of the following reasons.
Consider a case in which the master key of the system needs to be updated because the key is leaked to a hacker. In this case, the type of system (or product) or the like may be used as a unit of exclusion of a hacker apparatus. Thus, in order to update the master key, a large amount of information to a certain degree is required as a bundle of key information for identifying the master key for each unit. For this reason, the system information inevitably has a comparatively large size.
In addition, even if the possibility of existence of a defect such as an injury or dirt on a disc is taken into consideration, it is important to read out the shipping-time information with a high degree of accuracy from the reliability point of view. For this reason, disc information and system information are stored repeatedly. That is to say, the same data is recorded a plurality of times. Naturally, the amount of the shipping-time information cannot but increase.
Falsification of information must be avoided because, if the system information used for excluding an apparatus of a hacker as described above is not prevented from being falsified, the system information does not have a meaning. The function of the system information cannot be executed unless falsification of the system information is avoided effectively.
It is important for the shipping-time information as a prerecorded information to satisfy the above requirements. A recording technique suitable for the shipping-time information is also demanded.
It is to be noted that, as a method for prerecording the shipping-time information onto a disc, a technique of creating embossed pits on the disc is known.
If operations to record and reproduce high-density data onto and from an optical disc are taken into consideration, however, the embossed-pit-prerecording technique has problems.
For operations to record and reproduce high-density data onto and from an optical disc, a groove with a small depth is required. In the case of a disc manufactured by creation of grooves and embossed pits at the same time by using a stamper, it is extremely difficult to form the grooves and the embossed pits with the depth of the grooves made different from the depth of the embossed pits. Thus, the depth of the grooves and the depth of the embossed pits cannot help becoming equal to each other.
However, embossed pits with a small depth raises a problem that a signal having a high quality cannot be obtained from the embossed pits.
Assume for example that data having an amount of 23 GB (Giga Bytes) can be recorded onto and reproduced from an optical disc with a diameter of 12 cm and a cover (substrate) thickness of 0.1 mm through an optical system employing a laser diode generating a laser having a wavelength of 405 nm and an objective lens with an NA of 0.85 by recording and reproducing phase change marks at a track pitch of 0.32 μm and a linear density of 0.12 μm/bit.
In this case, the phase change marks are recorded onto and reproduced from a groove created to have a spiral shape on the disc. In order to suppress media noises caused by the high density of the phase change marks, it is desirable to create a groove with a depth of about 20 nm of a depth in the range λ/13 to λ/12 where notation λ denotes a wavelength.
In order to obtain a signal from embossed pits having a high quality, on the other hand, it is desirable to create a groove with a depth in the range λ/8 to λ/4. After all, it is impossible to get a good solution to the problem of providing the same depth to the groove and the embossed pits.
From this situation, there has been demanded a method of prerecording shipping-time information, which compensates embossed pits.